I have 2 favorites in mind: planks and Abdominal Hold. The usage of a timer in these exercises give me an incentive to try and beat my previous attempt or workout.
Planks are common to people, but abdominal hold is fairly new to me. It involves you sitting tall on a sturdy chair. Place your hands on the edge with your fingers pointing towards you knees. Tighten your abs, bring your toes 2-4 inches off the foor, and lift your butt off the chair. Hold that position for as long as you can.
It takes some getting used too, but I'm already getting better at it ;)
Oh, and try Flutter Kicks!
I recommend using YouTube to see what these exercises look like in order to see if you would like it, and stick to it, and to see if you're doing it properly.
Mika, flat stomachs don't happen by them selves. You got to put some work into getting 'em flat. Try planking, or sit ups. If you have access to a health club, try different "crunch" machines.